This invention relates to a method of producing a material for superconductor in which Cu and Nb or V are main components, and more particularly to a method of producing a material for superconductor which is extremely homogeneous and ductile.
Since superconductive wires comprising a compound such as Nb.sub.3 Sn or V.sub.3 Ga have a high critical current density under a high magnetic field, they are used as a wire for a superconductive magnet generating a large magnetic field.
Such a superconductive wire is manufactured mainly by a bronze method. However, the process of this method is complicated, and the superconductive characteristics with respect to strain of wire obtained by this method are extemely low. In recent years, superconductive wire in which discontinuous fibers are formed in a Cu base, a so-called in-situ superconductive wire, is being developed to provide a wire free from these defects. The characteristic of this in-situ superconductive wire resides in that extremely thin Nb.sub.3 Sn or V.sub.3 Ga filaments are dispersed discontinuously in a Cu alloy.
The basic method of manufacturing wires of this kind will now be described, taking as an example a method of manufacturing Nb.sub.3 Sn wires.
A mixture of Cu and Nb is prepared so that it contains 15-50% by weight Nb, and the mixture is melted and cast under vacuum or in an inert gas atmosphere to form a Cu-Nb material for superconductor. The Nb in this Cu-Nb material is substantially not solid-dissolved in the Cu base, the material has a structure in which Nb is precipitated in dendritic precipitates in the Cu. When the Cu-Nb precipitates are then subjected to an area-reduction process to form an elongated wire, the dendritic Nb precipitates are stretched and dispersed in the shape of discontinuous fibers in the basic Cu material. The surface of the elongated wire is then coated with Sn, and is thereafter subjected to a diffusion heat treatment at a temperature of 500.degree.-750.degree. C. which diffuses the Sn into the Cu base group so that it reacts with the Nb to form the compound Nb.sub.3 Sn dispersed in a Cu alloy.
However, in this method, the shape and size of the Nb dendritic precipitates in the Cu-Nb material are sensitive to cooling rate during the melting and casting, so that it is difficult to produce an ingot which is highly homogeneous in the longitudinal and diametrical directions thereof. When the Nb dendritic structure in the Cu-Nb material is uneven, the critical current density characteristics of the resultant superconductive wire are uneven in the lengthwise direction thereof.
The surface of the ingot is usually peeled off and the top and bottom thereof are cut off before the ingot is subjected to the area-reduction process, so that the yield is extremely low.
In one method of producing a Cu-Nb material for superconductor, which can solve this problem of the heterogeneous structure of a Cu-Nb material, powdered Cu and Nb are used. This method will now be described.
Cu powder and Nb powder are first mixed, and the resultant mixture is compression-molded and then sintered to obtain a Cu-Nb ingot. This Cu-Nb ingot is subjected to an area-reduction process to form an elongated wire, this wire is coated with Sn and is then subjected to a diffusion heat treatment to provide a Nb.sub.3 Sn superconductive wire.
According to this method, if the particle sizes of the raw Cu and Nb powders is regulated suitably, and, if the powders are then mixed by being agitated thoroughly, a substantially homogeneous Cu-Nb material for superconductor can be obtained.
However, the largest drawback encountered in this conventional method resides in that, since Nb is active, the raw Nb powder is easily contaminated with oxygen. Commercially-available, raw Nb powder has a high oxygen concentration of 0.1-0.5% by weight. If this raw Nb powder is mixed as it is with Cu powder, and the resultant mixture is subjected to mold-sintering to obtain an ingot, the area-reduction processability of the ingot is adversely affected as the ingot is stretched into a wire, so that extending the ingot into an elongated wire is difficult. This is thought to be because the Nb particles dispersed in the Cu base are contaminated with oxygen and are thus embrittled. Accordingly, when the ingot is stretched into a wire, the Nb particles are plastically deformed, so that stress concentrations occur in the Nb particles and generate cracks in the ingot. A Cu-Nb material for superconductor in which such oxygen-contaminated Nb particles are dispersed in the Cu base can not provide the resultant superconductor with the desired superconductive characteristics.
Therefore, according to the conventional method, it is necessary to subject the raw powder to a pretreatment such as a reduction treatment. This makes the production process complicated. In addition, in order to obtain a sinter in which Nb particles are dispersed uniformly in the Cu base, it is necessary to mix two different raw powder by thoroughly agitating them. This makes the production process even more complicated.
Further, Japanese Laid-open Patent Publication No. 56-162412 (1981) discloses a method of producing a superconducting wire using Nb particles coated with Cu by vapor or electrical deposition method. This method also needs compricated processes, for example, reduction treatment of the particles, complete mixing of the particles, a deposition process of Cu to the fine particles, etc.
The above is a description of a conventional method of producing superconductive wire, taking as an example a method of producing Nb.sub.3 Sn superconductive wire. The method of producing V.sub.3 Ga superconductive wire is similar to that of producing the Nb.sub.3 Sn superconductive wire.